The described embodiments relate generally to neutral point clamped (NPC) converter control systems and methods for balancing neutral point (mid-point) voltage. This type of voltage balancing is especially useful in renewable power generation systems such as wind and photovoltaic power generation systems, for example.
Neutral point clamped (NPC) converters are widely applied in power conversion systems. One challenge for a NPC converter topology is neutral point voltage imbalance. For optimal operation of a typical NPC converter, such as a three-level converter, the neutral point voltage imbalance should be maintained at about zero volts. Typically, a pulse width modulation (PWM) control strategy can be used to balance (center) the neutral point voltage.
For PWM balance control strategies, the basic principle is to inject an additional compensation command during PWM modulation. By this technique, the neutral point current is regulated to charge or discharge a capacitor bank to compensate for any capacitor voltage imbalance (neutral point voltage imbalance). A conventional power conversion system may include a source-side converter and a line side converter. In conventional methods, the additional compensation command is sent to only one of the source-side converter and the line side converter.
However, single side control strategies may not always be sufficient to balance a relative larger neutral point voltage imbalance of a power conversion system. In such situations, as can be seen with reference to the example graph of FIG. 1, a voltage trend curve A1 of an upper capacitor and a voltage trend curve B1 of a lower capacitor may increasingly diverge over time.
Therefore, it is desirable to provide a NPC converter control system and method to improve performance of balancing neutral point voltage.